Amplitude modulation (AM)-to-AM distortion and AM-to-phase modulation (PM) distortion have been considered as examples of nonlinear distortion occurring in a power amplifier used in a wireless system. Recently, the frequency bandwidths used in wireless systems have been continuously widened. As the bandwidths are widened, the impact of distortion caused by a memory effect, which is dynamic distortion dependent on a variation and a past history of signals input to the power amplifier, as well as AM-to-AM distortion and AM-to-PM distortion, are no longer ignorable. To improve power amplifier characteristics related to compensation of non-linear distortion, it has recently become important to understand how the power amplifier behaves under the memory effect and to correct distortion including distortion caused by the memory effect.
The memory effect is considered to be caused by a plurality of factors affecting each other. Examples of such factors include those caused by a member included in an amplifier circuit provided to the power amplifier, those dependent on frequency characteristic of a radio frequency (RF) circuit provided to the amplifier circuit, and those dependent on the frequency characteristic of a bias circuit that determines the operating point of the amplifier circuit.
Available as a model for expressing the memory effect that is dependent on the frequency characteristic of the RF circuit provided to the amplifier circuit is a Wiener-Hammerstein (W-H) type amplifier. The W-H type amplifier is an amplifier assumed to have a configuration in which linear time-invariant (LTI) filters are positioned before and after the amplifier circuit, respectively. Hereinafter, the LTI filter assumed to be positioned before or after the amplifier circuit are referred to as an “LTI”.
As a conventional technology for compensating for distortion caused by the memory effect, Japanese Laid-open Patent Publication No. 2009-200694 discloses a technology that uses predistortion, in which a signal to be transmitted is compensated using the inverse characteristic of nonlinear distortion of the power amplifier.
However, in a W-H type amplifier, a power spectrum that is measurable as an output from the amplifier module includes only amplitude information of the signal to be transmitted, and phase information is lost. Therefore, it has been difficult to estimate a frequency characteristic or a transfer function of the LTIs when a W-H type amplifier is used. Furthermore, it has been difficult to measure or to estimate the characteristic of the amplifier circuit and the characteristic of the two LTIs connected before and after the amplifier circuit in a separate manner.
Furthermore, in a power amplifier using predistortion, every parameter included in a lookup table (LUT) or the like are updated. Therefore, parameters that changes slowly by a small degree are also updated, and a complex algorithm is used in the compensation. Therefore, it is difficult to perform the compensation quickly.